Together with carbon/carbon composites, CMCs are thermostructural composite materials that are characterized by good mechanical properties that make them suitable for building structural elements, and by their ability to retain these mechanical properties up to high temperatures.
Thermostructural composites are used in particular in aviation and in space applications, and in particular for making parts of aircraft engines, or structural elements of space vehicles.
The manufacture of a part made of composite material generally comprises making a fiber preform to a shape that is close to that of the part to be manufactured, and then in densifying the preform with the matrix.
The fiber preform constitutes the reinforcement of the part to which it essentially confers its mechanical properties. The preform is obtained from fiber products such as thread, cloth, felt, etc. Shaping is performed by reeling, weaving, stacking two-dimensional plies of cloth or sheets of cables, . . .
Densification of the fiber preform with the matrix consists in filling the pores of the preform throughout its volume with the material that constitutes the matrix.
A first densification technique uses a liquid and consists in impregnating the preform with a liquid mixture that contains a precursor of the matrix material and then, optionally after drying and curing, in subjecting the impregnated preform to heat treatment in order to transform the precursor. Several consecutive cycles of impregnation and of heat treatment are generally necessary in order to achieve the desired degree of densification.
A second densification technique consists in infiltrating the preform with the material from which the matrix is made by chemical vapor infiltration (CVI). To this end, the preform is placed in an infiltration oven into which a gas is admitted. Under determined conditions of temperature and pressure, the gas penetrates into the core of the preform and, on contact with the fibers, the matrix material is formed by the gas decomposing or by component parts of the gas reacting.
In order to enable the fiber preform to retain the desired shape while chemical vapor infiltration is taking place, it is necessary at least during a first portion of the densification process, to hold the preform in tooling, generally made of graphite. Such solid tooling is expensive to make, in particular when the preform is complex in shape. It also needs to have numerous holes machined therein in order to provide the gas with access to the preform through the tooling. In addition, the tooling is heavy and bulky.
Unfortunately, chemical infiltration is a process that is generally very lengthy and very expensive. For example, a densification process typically requires several hundreds of hours. In addition, tooling that occupies an appreciable fraction of the working volume of the infiltration oven and having significant thermal inertia constitutes a drawback. Furthermore, matrix material is inevitably deposited on the tooling, with the consequence of large numbers of rejects due to the preform adhering to the tooling. Even in the best of cases, such deposits require the tooling to be renewed frequently.
Tooling is required during chemical vapor infiltration only until the preform has been consolidated. This stage is reached when a sufficient quantity of the matrix-forming material has been deposited to bond the fibers together throughout the volume of the preform so that after the tooling is removed the preform remains in the desired shape and can be handled. Densification can then be completed with the preform free from tooling. The tooling is nevertheless necessary during at least a portion of infiltration, and infiltration must be interrupted in order to enable the tooling to be withdrawn once the preform has been consolidated.
It is therefore desirable to be able to perform the entire chemical vapor infiltration process without it being necessary to hold the preform in tooling.
When the composite material has a carbon matrix, it is possible, prior to chemical vapor infiltration, to consolidate the preform by means of a liquid. The preform is impregnated with a precursor of carbon, e.g. a resin having a high coke content. The impregnated preform while held in tooling, also known as a "shaper", is dried so as to eliminate any solvent, and then the carbon-precursor resin is polymerized (cured) and heat treatment is performed to cause pyrolysis of the precursor and to leave a carbon residue that consolidates the preform.
An analogous consolidation technique could be devised for use with CMC. However, tests performed by the Applicant in which a fiber preform is consolidated by being impregnated by means of a liquid constituting a precursor of an organosilicon type ceramic, in particular polycarbosilane (PCS) as a precursor for silicon carbide (SiC), by using the conventional methods for cross-linking such precursors, have not given satisfaction.
CMC parts have been made from fiber preforms made of carbon or of silicon carbide, consolidated by being impregnated with a PCS solution, dried, cross-linked by the oxygen in the air, and heat treatment, and the consolidated preforms were densified by chemical vapor infiltration using silicon carbide. Parts made in this way demonstrate mechanical properties that are considerably less good than those obtained when consolidation is performed by chemical vapor infiltration.
This deterioration in mechanical properties appears to stem from the technique used for cross-linking the PCS. Uniform cross-linking throughout the volume of the preform is practically impossible to obtain, in particular when the preform is thick. As a result, a cross-linking gradient exists and zones may even be present where the PCS is not cross-linked, i.e. where it has not been made unmeltable, and as a result it takes up the liquid state during the heat treatment. Furthermore, it is necessary to use very strong tooling for holding purposes in order to counter substrate deformation due to the production of volatile species during pyrolysis. In addition, the presence of oxides in the ceramic residue runs the risk of putting a limit on the refractory properties of the CMC.
Other known techniques for cross-linking PCS, such as cross-linking by means of sulfur or by electromagnetic radiation, or by electron beam process, or by plasma treatment, cannot give satisfaction either, even if the inclusion of oxygen into the ceramic residue is avoided.
Sulfur may constitute a source of pollution. Use of radiation generally leads to long-duration treatment and, like electron beam process, requires an installation that is cumbersome and expensive. Finally, plasma treatment also requires an expensive installation and is effective over a limited thickness only.
An object of the present invention is thus to provide a method of manufacturing CMC parts in which the preform can be consolidated by liquid impregnation using a ceramic precursor, prior to being densified by means of a liquid or by means of chemical vapor infiltration, while avoiding the above-mentioned drawbacks, and without degrading the mechanical properties of the resulting parts.